In driving out the old staff, care should be taken to see that the hub in the balance arm has a secure seating in the taper mouth of the stump-the balance arm must not touch the stump; the hub should never be loosened in the arm.
Notwithstanding the accuracy with which the staffs are made, it will be found sometimes that the staff is too tight a fit for the hole in the hub. In such cases, the staff should be reduced with oilstone powder and a strip of soft steel, used by hand, while the staff is revolved in the lathe; or a pivot polisher lap may be used. The hand method is generally satisfactory, for the reduction required is but slight.
In driving in the staffs, care should be taken to seat the taper shoulder firmly; this requires a special punch, with outer diameter slightly less than the taper shoulder of the staff. If the young watchmaker will bear in mind the points we have mentioned, he should have no difficulty.